Buoyancy/Instability Acronyms in SPC
Forecasts

The SPC uses
nomenclature for Convective Available Potential Energy (CAPE) and
Lifted Index (LI) that identifies the lifted parcel being used in the
calculation. This is done because the magnitude of CAPE or LI can be
very sensitive to the parcel choice, and users of SPC forecast
products should have knowledge of what parcel is used in the
calculation of CAPE and LI. Even in cases of strong instability, the
values calculated from a sounding can vary considerably depending on
the parcel that is lifted (e.g., there can be a large difference
between using the most unstable parcel and the 100 mb AGL mean layer
parcel).

4. For parcels used in operational model forecast gridded fields,
CAPE or LI will be preceded by the acronym of the model being used,
such as NAM MUCAPE, etc. This is necessary because operational
models such as the NAM, GFS, and RUC may utilize different lifted
parcel definitions than those used in the SPC sounding analysis
program called N-SHARP.

Note the
first three parcel types are used on current diagnostic data as well
as with model forecast data available in the SPC N-AWIPS
workstations.

Current
Diagnostic Fields:

5. For current surface-based, mean-layer, and most unstable
instability values produced by the hourly SPC Mesoscale Analysis
fields (see: http://www.spc.noaa.gov/exper/mesoanalysis),
which are derived using real-time surface observations coupled with
hourly RUC model data above the surface, SBCAPE, SBLI, MLCAPE, MLLI,
MUCAPE, and MULI will be used without any modifiers. For current
diagnostic values of instability, it shall be assumed that the SPC
Mesoscale Analysis or current RUC sounding modified by representative
surface conditions is the source of these values.

Model
Forecast Fields

6. For forecast instability values based directly on model grids
or model soundings without any modifications, the nomenclature in 1
through 3 will be used preceded by the model acronym, for example,
NAM SBCAPE, NAM SBLI, GFS MLCAPE, GFS MUCAPE, etc.

7. For forecast instability values using model soundings that are
subjectively adjusted for expected surface conditions, the
nomenclature in 6 will be used, but any adjustments made will be
mentioned in the text of the discussion.

The above
usage procedures should cover most cases. However, if an SPC
forecaster wishes to use a different parcel method for assessing
instability in a particular situation (for example, the 700 mb parcel
may be chosen when the most unstable parcel is located above the
lowest 300 mb of a sounding), this information will be spelled out
specifically in the text of the discussion (e.g. ..."MUCAPE
based on lifting a parcel from 700 mb").

Further, for
all LI values, 500 mb will be considered the level of lifted
parcel-environment comparison by default. If an SPC forecaster uses
a different level of comparison (e.g., 300 mb) for the LI, that
information will be included in the text discussion.